1. Field of the Invention
The present invention relates to mass spectrometry and a mass spectrometer used for quantitative analysis and simultaneous qualitative analysis of trace amounts of compounds and for structural analysis of sample ions.
2. Description of Related Art
MALDI (matrix-assisted laser desorption/ionization) is one kind of method of laser ionization. This method uses a matrix (such as liquid, crystalline compound, or metal powder) having an absorption band at the used laser wavelength. A sample is mixed and dissolved in the matrix. Then, the matrix is firmly held on a sample plate. The matrix is irradiated with laser light to vaporize or ionize the sample. Since this is a pulsed ionization method using a pulsed laser, MALDI is compatible with time-of-flight mass spectrometers (TOFMS).
In recent years, a matrix-free ionization method capable of omitting a procedure for mixing a matrix and a sample has been developed. In many cases, a microscopic structure on a sample plate is used.
On the other hand, a TOFMS is a mass spectrometer for finding the mass-to-charge ratios of ions from the times taken for the ions to reach the detector after a given amount of energy is imparted to the ions to accelerate them such that the ions fly. In the TOFMS, the ions are accelerated by a constant pulsed voltage of Vα. At this time, from the law of energy conservation, the velocity v of the ions is given by
                                          mv            2                    2                =                  qeV          a                                    (        1        )                                v        =                                            2              ⁢              qeV                        m                                              (        2        )            where m is the mass of each ion, q is the electric charge of each ion, and e is the elementary electric charge.
The ions reach a detector in a flight time of T, the detector being in a position at a given distance of L.
                    T        =                              L            v                    =                      L            ⁢                                          m                                  2                  ⁢                  qeV                                                                                        (        3        )            It can be seen from Eq. (3) that the flight time T varies according to the mass m of the ion. TOFMS is an instrument for separating masses by making use of this principle. One example of a linear TOFMS is shown in FIG. 1. Furthermore, reflectron TOFMS instruments in which the energy focusing can be improved and the flight distance can be prolonged by placing a reflectron field between the ion source and the detector have enjoyed wide acceptance. One example of a reflectron TOFMS is shown in FIG. 2.
TOFMS has two kinds of methods of measuring flight times. In one method, ions created by an ion source are directly extracted with a high pulsed voltage, and the flight time is measured. This is known as the coaxial TOFMS. Frequently, this is combined with a pulsed ion source in use. In the second method, ions created by an ion source are transported with a kinetic energy of about tens of eV. The ions are accelerated with a high pulsed voltage in a direction perpendicular to the direction (axis) of the transportation. Under this condition, the flight time is measured. This is known as the orthogonal acceleration TOFMS. This is often combined with a continuous ion source in use.
An ionization method employing laser irradiation can use both of these TOFMS methods. In the case of the coaxial design, laser irradiation and measurement of flight time are synchronized. In the case of the orthogonal acceleration design, laser irradiation and measurement of flight time are not synchronized. See, for example, Japanese Patent Laid-Open No. H7-178070 and Japanese Patent Laid-Open No. 2005-134181.
MALDI is superior to other ionization methods presently used in mass spectrometry in its ability to ionize highly varied compounds. However, when a matrix material and a sample are mixed and crystallized, a spread of about 1 mm in diameter is often produced. Frequently, the sample is locally present within this area. Ionization is effected by laser irradiation. The diameter of the irradiating laser light is 100 μm, which is sufficiently smaller than the crystallized area. Therefore, there is the problem that the ion intensity varies greatly according to the beam position on the sample due to the localization of the sample. In particular, where the spectral intensity is plotted on the vertical axis of a graph (hereinafter referred to as the time trace) and the data acquisition time is on the horizontal axis, plural spectral peaks appear as shown in FIG. 3.
In an ordinary mass spectrometer, about hundreds to thousands of mass spectra are collected and accumulated. As a result, an accumulated mass spectrum with good signal-to-noise ratio (S/N) is obtained. Because spectra are accumulated randomly during the laser irradiation process, mass spectra with extremely low signal intensities are also collected. In total, the S/N may be deteriorated.